Graded index polymer optical fibers, and methods of making them, are known. Thus, a method of making a graded index polymer optical fiber comprising a spinning process employing spinning masses comprising a core spinning mass and a sheath spinning mass, the spinning masses comprising a polymer, a polymerizable monomer, and an initiator, wherein the monomer in the core spinning mass has a higher refractive index than the monomer in the sheath spinning mass, and wherein the spinning masses are extruded and cured, is known from Ho et al. in Polymer Journal, Vol. 27, 1995, pages 310-313. Ho et al. disclose a top to bottom spinning process for the preparation of a gradient-index polymer fiber by extruding a core spinning mass containing poly(methyl methacrylate) and benzyl methacrylate and a sheath spinning mass containing poly(methyl methacrylate) and methyl methacrylate into an enclosed diffusion zone which is maintained at 80.degree. C. and subsequent UV curing. The speed of this spinning process is up to 2.5 m/min. The obtained fiber, having a diameter of 1 mm, has a quadratic distribution of refractive index of only 62% of the diameter of the fiber. In the process of Chen, Ho et al. described in Journal of Applied Polymer Science, Vol. 60, 1996, pages 1379-1383, this fiber is subsequently extruded through a smaller orifice to remove 40% of the outermost portion of the fiber. The obtained fiber is to be used as an imaging lens. A disadvantage of the method of Ho et al. is that it fails to provide fibers with the desired refractive index profile suitable for the above-mentioned purpose. A further disadvantage of this method is that UV curing is performed using UV-C light (253 nm), which requires special precautions with respect to the safety of the work environment.
Several other methods are known which disclose the manufacture of fibers having a graded refractive index profile. European Patent Publication No. 451,266 discloses a bottom to top spinning process for the preparation of a fiber consisting of two or more layers. Polymers and monomers that are used in this method are, e.g., poly(methyl methacrylate) and methyl methacrylate, benzyl methacrylate, 2,2,3,3-tetrafluoropropyl methacrylate, and 2,2,3,3,4,4,5,5-octafluoropentyl methacrylate, or mixtures of these monomers. During diffusion volatilizing substances released from the fiber are removed by a stream of an inert gas such as nitrogen. Optical transmission elements with three or four layers and having a radius of 0.4-0.6 mm and a length of 13.5-18.4 mm are provided, which elements are used as lenses in an image transmitting array of a copying machine. The optical transmission element has a quadratic refractive index distribution in the range of 0.25-0.75 r.sub.o from the center of the element. A disadvantage of this method is that the speed of this bottom to top process is low, i.e., 0.5 m/min. Another drawback is that the evaporated monomers have to be taken care of, i.e., either processed as waste or recycled, which is detrimental to the economics of the process. It is desired to provide a fast and economical process in which the spinning occurs from top to bottom, providing a graded refractive index polymeric optical fiber having the desired refractive index distribution profile.
Similar methods and optical transmission elements are known from European Patent Publication Nos. 447,548 and 527,239. The method of European Patent Publication No.447,548 provides an optical transmission element consisting of three or more layers and having a quadratic refractive index distribution in the range of 0.25-0.70 r.sub.o from the center of the element. This method has the same disadvantages as described above for European Patent Publication No. 451,266.
The method of European Patent Publication No. 527,239 provides a plastic graded-index transmission element consisting of three or more layers, having a radius of 0.45.+-.0.1 mm, and possessing a quadratic refractive index distribution in the range of 0.25-0.8 r.sub.o from the center of the element. It is stated that when only a core and a sheath spinning mass mutually varying in refractive index are used, it is difficult to make a fiber having a quadratic refractive index distribution in the range of 0.25 to about 0.70 r.sub.o from the center of the fiber. Therefore, in this method, from a performed fiber an outer peripheral portion is eliminated. The resulting fibers are used as lenses in copiers and facsimiles. Apart from being a bottom to top process in which monomers are evaporated from the fiber, the handling of fibers having a radius smaller than 0.35 mm also presents a problem.
Japanese Patent Publication No. 96/ 106,016 discloses a method of manufacturing a plastic optical fiber which can be used as an optical information communication medium. Each spinning solution comprises a polymer, a polymeric monomer, and a nonpolymeric compound. The addition of the non-polymerizable compound to the spinning solution is essential for obtaining a continuously distributed refractive index. This process suffers from the same drawbacks as described above for European Patent Publication No .451,266. It is a bottom to top process and the non-polymerizable compound is partially volatilized. A further disadvantage of this process is that the non-polymerizable compound remains in the fiber as a plasticizer and may lead to instability as a result of further diffusion of the low-molecular weight compound. Similar methods have been disclosed in Japanese Patent Publication Nos. 96/106,013 and 96/ 106,014.
Further, a method comprising a bicomponent spinning process employing a core spinning mass and a sheath spinning mass is disclosed in Japanese Patent Publication No. 94/ 186,441. A refractive index distribution type plastic optical transmission body is produced by discharging a polymer monomer mixture determined by a 50-90% conversion from the outside discharge hole of a nozzle plate with a double concentric discharge hole, while simultaneously discharging a monomer mixture or a polymer monomer mixture of conversion 50% or less from the inside discharge hole. The latter mixture contains a non-polymerizable colorless transparent compound with a high refractive index, such as benzyl butyl phthalate. Said mixtures are discharged to produce a core--sheath composite rod. The high refractive index compound is diffused from the mixture in the inner layer (the core mixture) into the mixture in the outer layer (the sheath mixture). Further polymerization of the discharged rod results in an optical transmission body with a refractive index distribution which continuously decreases from the center toward the periphery. To prepare an optical fiber from the rod, the rod is drawn to the desired fiber diameter.
A disadvantage of this method is its multi-stage character. The separate steps of discharging a rod, post-polymerizing the rod, and drawing it to a fiber, make for an inherently not fully economical process. Also, the step of forming the graded index rod is slow. It is desired to provide a process which yields a fiber directly, i.e., without first having to form a rod to be drawn. It is further desired that such a process be a one-step, preferably continuous process.
Apart from its objectionable multi-stage character, this process has a further drawback in that the non-polymerizable high refractive index compound, which is miscible with the sheath and core polymers, remains in the system as a plasticizer. Further, the presence of such a low-molecular weight compound means that the eventual optical fiber may be prone to temperature induced instability as a result of further diffusion of the low-molecular weight compound. It is therefore desired to provide a graded index polymeric optical fiber where the refractive index distribution is fixed in the polymer.
Methods of providing polymeric optical fibers where the refractive index distribution in the eventual fiber is not determined by the presence of a low-molecular weight compound are known.
In this respect, Japanese Patent Publication No. 90/ 33,104 is referred to which discloses a method of making a graded index polymer optical fiber, again by means of bicomponent spinning of a core and a sheath spinning mass. The sheath is spun from a mixture of a polymer, e.g, poly(methyl methacrylate), and a monomer, e.g., methyl methacrylate. The core is spun from a high refractive index monomer, e.g., phenyl methacrylate. After having been discharged through a bicomponent spinning nozzle, the spun core-sheath filament is passed through a heated tube to effect diffusion and then subjected to UV irradiation to effect photopolymerization. Thus, a core-sheath optical fiber is formed in which, due to diffusion of the methyl methacrylate and phenyl methacrylate monomers, a refractive index distribution is attained. The index distribution is fixed in a polymeric structure as a result of the polymerization.
This process is not carried out at the desired rate either, typical residence times in the heated tube and during irradiation being about three minutes each. Further, the process has a drawback in that the core does not contribute to the filament forming.
In Japanese Patent Publication No. 91/ 42,604 a different method is disclosed. Here, a graded index polymer optical fiber is made by bicomponent spinning, the core material comprising a polymer-monomer mixture and the sheath material being a low refractive index polymer, e.g., poly(tetrafluoropropyl methacrylate). This method also requires a too long process time, again involving a heated tube and UV irradiation.
For carrying out the methods described in Japanese Patent Publication Nos. 90/ 33,104 and 91/ 42,604, a bicomponent spinning apparatus is used in which the spinning masses are discharged from the bottom to the top. For the sake of practicality it is desired to provide a process for making graded index polymer optical fibers in which the introduction of the refractive index distribution is sufficiently fast to allow normal spinning (in which a spinning mass is discharged from the top to the bottom of the spinning apparatus).
Several other methods of making graded index polymer optical fibers have been disclosed in the art. Thus, Japanese Patent Publication No. 77/ 5857 pertains to a method in which a rod is formed by incompletely polymerizing a high refractive index monomer (diallyl phthalate). A monomer having a lower refractive index (methyl methacrylate) is diffused into the rod under simultaneous polymerization. A similar method is disclosed in Japanese Patent Publication No. 81/ 37521. A partially polymerized high refractive index polymer is placed in a monomer which is in the gaseous phase, and this then diffuses into the polymer with further polymerization taking place.
Japanese Patent Publication No. 88/ 94,228 discloses a method wherein a high and a low refractive index spinning mass containing a polymer, a vinyl monomer, and a photosensitizer are mixed and spun into a coaxial multilayer graded index optical fiber, with part of the vinyl monomer being evaporated from the spun fiber. This method is not based on diffusion of monomers with different refractive indices.
European Patent Publication No. 615,141 describes a method comprising melting a transparent polymer, injecting into the central portion of said melt a transparent and diffusible material having a refractive index different from that of said transparent polymer or a material containing said transparent and diffusible material and a transparent polymer, and performing an extrusion melt molding to form a plastic optical transmission medium. In this method, neither the core nor the sheath spinning mass comprises a polymer as well as a polymerizable monomer.
In Japanese Patent Publication No. 79/ 30301 two different monomers are heterogeneously copolymerized so as to form a gradient in the monomer composition. In Japanese Patent Publication No. 86/ 130904 a mixture of monomers having different refractive indices is subjected to curing from the outside. The monomer mixture on the inside becomes enriched with the higher refractive index monomer.
In European Patent Publication No. 208,159 a polymer monomer mixture is extruded, the monomer being volatile and having a relatively high refractive index. Vaporization from the outside leads to the core having a higher refractive index monomer than the sheath. A similar method using a vinylidene fluoride type resin and a methacrylate type monomer has been disclosed in Japanese Patent Publication No. 96/ 106,019.
In European Patent Publication No. 496,893 a poly(methyl methacrylate) tube is made and filled with a monomer mixture of methyl methacrylate and benzyl methacrylate. A graded index rod is formed by rotating the tube (about 20 hours) under polymerization conditions. The rod is drawn to a graded index polymer optical fiber. Also in European Patent Publication No. 497,984 a polymeric tube is made and filled with monomer. A rod having a graded index of refraction results from diffusion. A graded index polymer optical fiber is drawn from the rod.
All of these background art disclosures are unattractive methods which do not satisfy the aforementioned desires. The invention seeks to overcome the above problems and to provide an efficient method of making a graded refractive index polymer optical fiber which can be carried out as a fast continuous spinning process (preferably even up to 10 m/min or faster) and which provides an optical fiber having the desired round shape and refractive index profile.